The reapers have returned from dark space. With Earth under attack and the mass relays in a state of chaos from refugees and the councils fleet engaging reaper forces the galaxy is a mess. The outcome is unknown, the warfare is devastating. The galaxy will never be the same.

Cerberus are able to construct the SR-2 frigate class.
The Council will have one SR-2 (Normandy) starting unit (from what I've heard), but are unable to reconstruct it if it's destroyed. They can only make SR-1, which is smaller and weaker.